The good folks at ASAP Science posted an interesting video titled "Can You Trust Your Eyes?" In it, the narrator demonstrates how colors and tones will appear lighter or darker depending on what tone they're next to. As the video so eloquently illustrated, this phenomena - known as the Munker White illusion - can make tones that are actually the same brightness appear to be vastly different.

The "A" column appears darker than the "B" column,but in actuality, they are the same tone.

I teach a process for shooting in manual mode that depends heavily on the shooter's ability to analyze tones (covered in my How to Shoot in Full Manual online course here). So this video got me thinking about how we are basically doomed to make metering errors in our photography purely by nature of this illusion. No matter how experienced one may be in the manual metering process I teach or the somewhat similar Zone System by the great Ansel Adams, you will never escape this inherent failure of our eyes to accurately analyze tones.

Understanding this effect may not be vital information for the manual shooter, but it certainly couldn't hurt. For instance, let's say I'm spot metering off of a section of granite rock on a sunlit mountain side. And let's say the spot I'm choosing to meter off of is immediately adjacent to a dark patch of trees. In this situation I may analyze the tone of that granite to be about a +2/3 on my meter - in other words, I think that granite is just a shade lighter than middle tone. So I point my meter up to the mountain, line my meter up at +2/3, then recompose and take my photo. Everything comes out great.

But what if this spot was immediately adjacent to something light rather than that dark patch of trees? What if instead of the dark trees, there's a big swath of bright white snow next to it? According to this Munker-White illusion, I'll perceive my metered area as a different tone. When it's next to the trees, I perceive it as a shade lighter than middle tone (+2/3), but when it's next to the snow, I might perceive it as a shade darker than middle tone (maybe -2/3 or so). Of course, the rock hasn't changed its tone, it's purely my perception of the tone dependent on what tone it's next to.

Simply depending on what tone it's next to, I'll perceive my metered area differently and, thus, get a different exposure.

So this information may not fall in to the category of "photography tips" for everybody, but if you're an adroit user of your camera's light meter, you may want to be aware of this illusion. Try to block out the tones around your chosen spot meter area to avoid the trickery of this illusion. Here's one way to do it: just form your hand into a circle (like you're holding a pipe) then look through it like a telescope pointed at your chosen metering area. This will block out the surrounding tones and allow you to analyze the tonality of the subject untainted by the Munker-White illusion. Or if you don't want to look ridiculous, just spot meter off of tones that cover sufficient area to fill your viewfinder.